fix(x-client): mock external API calls in x-client tests

Tests were making real HTTP calls to api.x.immutable.com, causing
failures in CI: starkCurve tests got undefined responses, and imx
tests crashed Jest workers due to circular req/res references in
Axios responses.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

packages/x-client/src/imx.test.ts

@@ -37,14 +37,22 @@ describe('IMXClient', () => {
   });
 
   it('should instantiate a PRODUCTION IMXClient', async () => {
+    let axiosRequest: AxiosRequestConfig = {};
+    const adapter = jest.fn().mockImplementation(
+      async (request: AxiosRequestConfig) => {
+        axiosRequest = request;
+        return { status: 200 };
+      },
+    );
+
     const config = new ImxConfiguration({
       baseConfig: { environment: Environment.PRODUCTION },
     });
     const { assetApi } = new IMXClient(config);
-    const assetsResponse = await assetApi.listAssets();
+    const assetsResponse = await assetApi.listAssets({}, { adapter });
 
     expect(assetsResponse.status).toEqual(200);
-    expect(assetsResponse.config.headers?.['x-sdk-version']).toContain(
+    expect(axiosRequest.headers?.['x-sdk-version']).toContain(
       'ts-immutable-sdk',
     );
   });

packages/x-client/src/utils/stark/starkCurve.test.ts

@@ -9,6 +9,9 @@ import {
 } from './starkCurve';
 import { grindKey } from './legacy/crypto';
 import { createStarkSigner } from './starkSigner';
+import { getStarkPublicKeyFromImx } from './getStarkPublicKeyFromImx';
+
+jest.mock('./getStarkPublicKeyFromImx');
 
 describe('Key generation', () => {
   it('should generate random Stark key', async () => {
@@ -114,6 +117,13 @@ describe('Key generation', () => {
     ];
     tests.forEach((test) => {
       it(test.name, async () => {
+        const isAccountNotFound = test.name.includes('account not found');
+        (getStarkPublicKeyFromImx as jest.Mock).mockResolvedValue(
+          isAccountNotFound
+            ? { starkPublicKey: '', accountNotFound: true }
+            : { starkPublicKey: test.wantPublicKey, accountNotFound: false },
+        );
+
         const expectedStarkPubKeyBN = new BN(
           encUtils.removeHexPrefix(test.wantPublicKey),
           16,